1. Field of the Invention
This invention relates to reproduction of photographic images, and more particularly, to electronically generating a screened image, which at any point can be either black or white, but not an intermediate gray level. Such devices include thermal transfer fax machines, laser electrostatic, and inkjet printers. The invention further relates to adjusting the coarseness of the resulting screened image.
2. Description of the Related Art
A typical electronic device for reproducing photographic images consists of a scanning module, a screening module, and a marking module. The scanning module is used to sense the gray shade of each point of the original photographic image, and report it in electronic form. The screening module processes this data into a form suitable for marking. Because many marking devices can only reproduce black or white at any given point, and not intermediate shades of gray, the screening module must generate a screened image containing only black and white points. Electronic signals representing the screened image are then routed to the marking module, which marks a medium such as paper or photographic film with the black and white points corresponding to the image generated by the screening module.
One technique used in a screening module is electronic simulation of a conventional screening technique. The conventional technique is described in U.S. Pat. No. 498,127, "Screen For Making Photomechanical Printing Plates." A state-of-the art electronic simulation is described in U.S. Pat. No. 4,012,584. This technique simulates gray shades by varying the size of the dots. However, the number and position of the dots remains constant. When used with marking modules exhibiting low spatial resolution, this technique suffers two problems. First, the screen patterns are coarse. Second, the number of shades that can be distinguished is small, also causing degradation in the reproduction quality. Conventional techniques which vary the size of the dots have not employed adaptive or recursive methods.
Conventional screening techniques have been employed with repeated cells of pixels in fixed locations. Often these fixed cells are rotated to enhance the pattern to the viewer. Since fixed cells have a finite number of pixels, it is only possible to obtain a limited number of gray shades. For example, where thirteen pixels are in a cell only thirteen shades of gray are possible. This is because each of the 13 pixels must be either on or off.
Adaptive dither is another technique that can be used in a screening module. With this technique, gray shades are simulated using very small dots. Lighter shades are represented with fewer dots than darker shades. An early example of this technique is given in U.S. Pat. No. 1,790,722 "Duplex photo modulator." Other popular examples are shown in Floyd, R. W., and L. Steinberg, "An Adaptive Algorithm For Spatial Grayscale", Proc. SID, vol. 17/2, pp. 75-77 and Ulichney, R, "Digital Halftoning", pp. 279-283. Adaptive techniques seek to cause an error signal representing the difference between the screened output and the input to approach zero. Typically, adaptive techniques generate a screened image exhibiting a large number of very small uniform size dots.
While the adaptive technique often provides better detail reproduction, less objectionable patterns, and a larger number of distinguishable shades of gray than the conventional screening technique, it has the further disadvantage of generating screen patterns that are too fine to be well reproduced by most marking devices. This problem is especially severe in reproducing gray shades in the 50% gray region.
As discussed above nothing in the state of the art suggests varying the size of dots generated by an adaptive technique.